A known eye-testing method for determining under-correction and over-correction of spectacle lenses or contact lenses, while distinguishing between far-sightedness, normal vision, and near-sightedness, disposes in a side-by-side arrangement a red visual target that displays a viusal fixation mark shaped as a black cross (plus sign) or a circle on a red background as well as a green visual target that displays a visual fixation mark identical to that of the red visual target on a green background, so that the test subject can see both targets simultaneously. This testing method makes use of chromatic aberration of the eye, using the fact that the difference in wavelengths between the red light and green light causes a displacement of the positions at which each of the red light and the green light come to a focus.
This testing method is described below.
When parallel reference rays of white light enter the eye through the crystalline lens thereof and come to a focus on the retina thereof, red light rays of a long wavelength are focused at a focal point behind the retina and green light rays of a short wavelength are focused at a focal point in front of the retina. In this state, because the distances of the focal point for red light and the focal point for green light from the retina are substantially equal, the visual fixation marks on the red and green backgrounds appear to have the same level of clarity. In other words, with a normal-sighted eye or optically-corrected eye, in theory the fixation marks of the red visual target and the green visual target appear approximately the same.
With a near-sighted eye, however, the reference light that has come from a distance is focused at a focal point which is in front of the retina, as will be described later with reference to the drawings, and thus the focal point for red light is closer to the retina than the focal point for green light, so that the fixation mark for the red visual target appears clearer than the fixation mark for the green visual target.
With a far-sighted eye which does not have the ability of accommodation, on the other hand, the reference light (parallel light) that has come from a distance is focused at a focal point which is behind the retina, as will be described later with reference to the drawings. When such an eye views the targets, the difference in wavelength causes that the focal point for the green light is closer to the retina than the focal point for red light, so the fixation mark for the green visual target appears clearer than the fixation mark for the red visual target.
In physical optics, the above theory holds for an eye without a crystalline lens after a cataract operation, an eye into which a lens has been inserted, and an eye of an elderly person of 70 years or more who has virtually no accommodation.
In physiological optics, however, it is rare for a far-sighted or normal-sighted eye with accommodation powers to see in accordance with the above theory.
This is because the human eye is constantly performing accommodation of the crystalline lens so that it is not fixed as in an artificial physical optical system. Since the human eye is focussing on objects in the outer world at a limited distance both physiologically and defensively, from the instant at which the eye opens, it is usual for even a far-sighted eye to cause the crystalline lens thereof to swell to focus on objects that the human wishes to see, at the instant at which the eye opens (within 0.5 to 1 second). In other words, a test subject is capable of clearly seeing both of the fixation marks of the red and green visual targets, by adjusting the crystalline lens unconsciously.
In addition, there is a tendency for the crystalline lens to retain this swelling state, which is an accommodation-rest state, rather than to retain a non-accommodated state (an alert state). Therefore, in most cases, both a normal-sighted eye and a far-sighted eye will tend to see the fixation mark of the red target more clearly.
At present, it is common to include red and green visual targets in an eye-chart for distance vision (5 meters), based on generic standards of the International Congress of Ophthalmology, but red and green visual targets have been virtually unused for a long time in ocular refraction testing, and, even when they are used, they are merely nominal since the determination thereof is unreliable. However, the spreading popularity of contact lenses in recent years is making it common for red and green visual targets to be used during the selection of contact lenses recently, starting with a check for over-correction by red and green viusal targets that is performed during the final stages of filling out a contact lens prescription.
However, as described above, the fixation mark of the red visual target tends to be seen clearly even with an over-corrected eye, so that easy use of red and green visual targets leads to problems such as a worsening in near-sightedness caused by over-correction of contact lenses and spectacles, and also an extremely high probability of causing VDT fatigue.
The present invention was devised in order to solve the above described problems, and thus an object thereof is the provision of an eye-testing method and an eye-testing apparatus using that method, wherein red and green visual targets are used in a state in which the unconscious accommodation of the crystalline lens performed by the test subject is suppressed as far as possible.